John Weaver, a 13-year veteran of the Washington Navy Yard, could not believe his eyes as he watched Aaron Alexis methodically shoot six of his friends where they worked -- killing five of them.

"I heard a really loud bang," he told ABC News exclusively. "Two seconds later, I heard another really loud bang and then I realized somebody's in here and they are shooting people. ... So I popped my head up and I saw him pointing his gun at my friend, and he shot her."

Weaver ducked behind his cubicle, pulled a cabinet to block the entrance and, while hiding beneath his desk, called 911.

The gunfire did not stop, as Alexis, a former Navy reservist, continued his rampage on building 197 at the Navy Yard on a mission to murder.

"I couldn't see [my friend] because she was behind her cubicle and we're like kind of caddy cornered. So I drop back down," Weaver said.

"Couple more shots went off, and this is with a shotgun. He was using a shotgun the whole time. I saw him with the shotgun," he added. "And I couldn't believe it. ... He was proceeding to just walk through the building, and just slaughter people. I was like, I couldn't believe it. ... I probably heard 30 shots.

"Then the fire alarm went off and that was my signal to get out of there," Weaver said. "So I jumped up, pushed the cabinet away ran away, went down to the end of my corridor and Mary was lying on the ground."

Mary Frances Knight was John Weaver's boss. As he looked at her, he knew she was dead.

"He looked like he was all business except he had his jaw clenched so you could just see the ball of his jaw, you know?" Weaver said. "And he, when he looked at her, he looked straight at her, clinched that jaw and just, boom. Like, you know, it was kind of like just business to him, it seemed."

As Weaver made his escape, he saw another co-worker covered in blood.

"I couldn't believe she was there. And I said to her, 'Um, are you OK?' And she said she got shot," he said. "And I look at her finger and she was missing about [half of it]. ... And then I said, 'Where else?' She said the back of her head, and I looked. It was just a scrape wound. So I told her she was OK and let's go down the stairs."

As they ran down the stairs, Weaver yelled for people to run, warning them of the gunman. Weaver bolted out the front door warning a police officer of the "man upstairs with a shotgun."

Weaver recalled his friends, Sylvia Frazier and 73-year old John Roger Johnson, "all those people that got killed who were the nicest people in the world.

"I mean, he was a great guy," Weaver said of Johnson. "I, like, came around the corner that morning and I tried to poke him in the eyes, and he gave me 'The Three Stooges.'"

"I mean, out of the six people that I know that he shot, he killed five of them," Weaver said. "And then I all tell you this. Though there could've been a lot more people killed there because he came in too early, because it was like 8 o'clock and a bunch of people were downstairs."

"Anybody he saw he shot," Weaver said.

Weaver told ABC News he didn't know if Alexis was targeting anyone that day and that he had never seen him before, but that he did work on Navy Marine Corps Internet (NMCI).

"He was on the tech refresh team, which means that you get a new computer every three years and he comes and gives you a computer," Weaver said. "My supervisor, I asked him, I said, 'Do you know the guy?' He said, 'Yeah.' I said, 'Did you guys reprimand him or do anything like that to him?' And he said no. He said, in fact, when he dealt with him, 'cause he thinks he told me that he had been deployed on our site before, and he said he was extremely pleasant to deal with and [he] had no issues with him whatsoever.

"These guys are like short-term contract workers," Weaver said. "So we don't really you know who they are."

In total a dozen innocent people were killed that early Monday morning -- a harsh reality that still had Weaver trembling as he talked.

"The people he killed were so nice," Weaver said. "They did not deserve to die a horrible death like they did.

"It was tragic. And I feel so bad for all of those people and their families, and what they're going to go through. "